The generation of synthesis gas by the high temperature partial combustion of a carbonaceous fuel, wherein the fuel is an ash containing petroleum coke, normally results in the production of a quantity of a usable gas together with a slag residue. The slag is usually comprised of a variety of compounds, including compounds of certain metals such as iron, nickel and vanadium, depending upon the composition of the coke feedstock.
Unfortunately, the presence of some of these metals introduces severe operating conditions into the gasification process, particularly when the metals are in their free state. This happens because, at the high temperatures found within the gasification process, the various metals present in the residue slag can take a form which will react unfavorably with certain components of the process equipment. An example of this is the high temperature ceramic refractory materials used for gasifier combustion chamber linings and for combustion chamber thermowells.
In the above mentioned gasification process, a high pressure gasifier is fed a pressurized stream of pulverized coke. The coke is combusted in the gasifier combustion chamber at a temperature within the range of 2000.degree. F. to 3200.degree. F., and preferably between 2500.degree. F. to 2800.degree. F., and at a pressure of approximately 5 to 250 atmospheres.
To adequately control the gasification process and ensure the safe operation of the process equipment, it is necessary to monitor the temperature inside the refractory lined gasifier combustion chamber. Among the most commonly used methods is to insert one or more thermocouples into the combustion area through a flanged opening in the gasifier vessel wall.
These process thermocouples are usually fabricated from commercially available noble metal thermocouple wire pairs, such as type B platinum/rhodium wire pairs. The wires are electrically insulated from each other by high temperature ceramic material such as alumina or magnesia. The insulated wire pair is enclosed in some form of protective sheath which can be made from ceramic or metal and which permits corrosive process gases from coming into direct contact with the wires.
Due to the generally aggressive nature of the residue slag, the sheathed thermocouple is placed inside a thermowell. The latter is usually constructed from material having a greater resistance to slag attack than does the thermocouple sheath.
The entire assembly is inserted into the gasifier via a hole drilled through the refractory lining. Thus, the tip of the thermowell is flush with the inner suface of the reactor combustion chamber. The hole in the vessel refractory lining communicates with a flanged opening in the vessel wall. Said opening permits passage of the thermocouple wire pair through a pressure seal fitting for connection with appropriate temperature display instrumentation.
Thermocouple failure is a frequent occurrence under the severe conditions encountered in a gasification reactor. Failure usually occurs when the thermocouple circuit formed by the wire pair is damaged. This results either by reactive species contaminating the wires, which leads to an error in the temperature indication, or by molten slag destroying a section of one of both wires. The latter causes the thermoelectric circuit to either open or to short-circuit.
For such damage to occur, molten slag, or a given reactive species, must first penetrate the ceramic refractory thermowell. It must then span the gap between thermowell and thermocouple protective sheath, and finally penetrate the protective sheath and insulation surrounding the wires.
Slag penetration will normally occur in one of two ways. The aggressive, molten slag can work its way through the refractory thermowell by corrosion, erosion and/or diffusion. Alternatively, the slag can rapidly move into the thermowell through cracks resulting from thermal shock. Such shocks can occur during upset conditions, or when the gasifier is started up or shut down.
Slag penetration via the first mechanism is particularly severe during coke gasification where the extremely corrosive slag easily penetrates even the most slag resistant refractory thermowell materials. For example, analysis of damaged thermowells made from Zirchrom 60, a commerically available refractory material, showed that coke slag has penetrated along minute cracks and grain boundaries. Once inside the thermowell, certain components of the coke slag, free iron in particular, migrated towards the thermocouple and reacted with the platinum protective sheath and thermocouple wires. such contact usually results in failure of the thermocouple.
It has also ben experienced that, upon occasion, even certain components of the ceramic refractory thermowell will migrate out of the thermowell and into the thermocouple. Here they will react with the wires at the high temperatures found inside the combustion section of a coke gasifier. It has been found from experience that even with the best available materials, daily replacement of damaged thermocouples is not uncommon.
To overcome the pervading difficulty encountered in gasifier operation, there is provided, in brief, a thermocouple assembly capable of functioning in the high temperature, high pressure, aggressive environment normally found inside an operating coke gasifier.
The thermocouple assembly is constructed with a noble metal (such as platinum) wire pair, such as a type R or type B wire pair, having an ungrounded junction, and which is insulated by dense magnesia. The noble metals referred to include, but are not limited to, gold, platinum, paladium, rhodium and ruthenium. The insulated wire pair is provided with a gas tight protective sheath of noble metal or noble metal alloy, such as pure platinum or platinum/rhodium alloy, which will not react with most of the aggressive constituents found inside the gasifier.
Surrounding the sheathed thermocouple is a thermowell made of at least two concatenated tubualr segments of refractory material capable of slowing the movement of slag components towards the thermocouple. The density, porosity, coefficient of thermal expansion and thermal conductivity of each of the segments is matched with those of the surrounding layers of gasifier refractory material. Thus, the thermowell will be less susceptible to thermal shock cracking and rapid invasion by slag.
The space between the sheathed thermocouple and the refractory thermowell defines an annular passage for conducting a stream of a purging and oxidizing and/or sulfiding gas which forms a dynamic envelope about the thermocouple. The flow rate of purge gas is controlled so that it is sufficiently high to maintain a critical partial pressure of oxygen and/or sulfur. The flow rate is low enough, however, so that the flow of gas does not introduce measurable error into the measured temperature indication as a result of convective cooling of the thermocouple junction.
Functionally, when the deleterious components of the molten slag, such as metals in the free state, and metallic iron in particular, eventually penetrate the thermowell, they will contact and react with the oxidizing and/or sulfiding gas prior to reaching the interior parts of the thermocouple assembly. Once in the oxidized or sulfided form, the aggressive components, and iron in particular, will have been neutralized and will no longer be able to attack and destroy the thermocouple protective sheath or the thermocouple wires. The oxidized and/or sulfided components are then swept out of the annular passage by the dynamic motion of the gas purge.
It is therefore an object of the invention to provide a thermocouple assembly which is capable of resisting physical deterioration when exposed to a hostile, high temperature environment.
A further object is to provide a thermocouple of the type described which is capable of functioning in the high temperature environment of a coke gasifier, by neutralizing the effect of the reactive elements in the slag which would otherwise destroy the thermocouple.
It is a still further object to provide a thermocouple of the type just described which is capable of resisting slag penetration by constructing the refractory thermowell in a manner which is less susceptible to thermal shock cracking and rapid slag invasion.